Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal (hereinafter referred to as a lithium-containing composite oxide) such as LiCoO2, LiNi1/3Co1/3Mn1/3O2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
A lithium secondary battery using LiCoO2 among such lithium-containing composite oxides as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a cathode active material, further improvement of the capacity density per unit volume of a positive electrode layer and the safety, has been desired. On the other hand, there has been a problem of deterioration of the cyclic properties such as gradual reduction of the battery discharge capacity due to repetitive charge and discharge cycles, a problem of the weight capacity density or substantial reduction of the discharge capacity at a low temperature.
In order to solve such problems, Patent Document 1 proposes to replace 5 to 35% of Co atoms in LiCoO2 with W, Mn, Ta, Ti or Nb to improve the cyclic properties. Further, Patent Document 3 proposes to use hexagonal LiCoO2 as a cathode active material to improve the cyclic properties, wherein the c axis length of the lattice constant is at most 14.051 Å, and the crystal lattice size in (110) direction of the crystal lattice is from 45 to 100 nm.
Further, Patent Document 2 proposes to produce a Li—Ni—Co composite oxide or a Li—Ni—Mn composite oxide by pulverizing a coprecipitate of Ni and Co, or Ni and Mn, then drying and granulating it by a spray dryer, and adding a powder of a lithium compound thereto, followed by mixing and firing.
Further, Patent Document 3 proposes a method for producing a lithium composite oxide, wherein a nickel material, a cobalt material and a manganese material are wet-pulverized, the obtained pulverized product is granulated by spray drying, the obtained granulated product is further dry-mixed with a lithium material, and the obtained dry-mixed product is fired to obtain a Li—Ni—Co—Mn composite oxide.
However, also in this case, a cathode active material having a high volume capacity density can hardly be obtainable, and the method is inadequate also with respect to the cyclic characteristics, safety and large current discharge characteristics.
Further, Patent Document 4 proposes a process for producing a cathode active material for a lithium secondary battery, which comprises a step of mixing a lithium source, a metal source and a doping liquid in the form of a solution or suspension containing a doping element, and a step of heat-treating the mixture.
Patent Document 5 proposes a process for producing a cathode active material for a lithium secondary battery, which comprises adding a metal starting material to a coating liquid in the form of a solution or suspension containing a doping element soluble in water or in an organic solvent, thereby to have the metal starting material surface-treated, then drying it to prepare a precursor, mixing it further with a lithium starting material, followed by heat treatment. In its specific embodiment, a doping element solution using an organic solvent, or an aqueous suspension solution containing a doping element made of boron, is used as the doping liquid.
Further, Patent Document 6 proposes a process for producing a cathode active material for a battery, which comprises treating particles of a metal salt with a colloidal aqueous coating solution obtained by adding water to (NH4)2HPO4 and Al(NO3).3H2O, followed by drying, then mixing with a lithium compound and heat treatment. However, the obtained cathode active material is unsatisfactory in the battery performance i.e. the discharge capacity, the durability for charge and discharge cycles and the safety.
As described above, in the prior art, with respect to a lithium secondary battery employing a lithium composite oxide as a cathode active material, it has not yet been possible to obtain one which sufficiently satisfies all of the volume capacity density, the safety, the coating uniformity, the cyclic properties as well as the low temperature characteristics and the production costs.
Patent Document 1: JP-A-3-201368
Patent Document 2: JP-A-10-134811
Patent Document 3: JP-A-2005-123180
Patent Document 4: JP-A-2002-373658
Patent Document 5: JP-A-2003-331845
Patent Document 6: JP-A-2003-331846